1. Field of the Invention
The present invention relates to an automatic wire extension method for a wire cut electric discharge machine.
2. Description of the Related Art
Generally, wire cut electric discharge machines comprise upper and lower wire guides, respectively disposed above and under a worktable on which a workpiece is fixed and individually formed with a wire passage for guiding a wire electrode (hereinafter referred to as wire). These machines are arranged to generate an electric discharge between the wire and the workpiece, while delivering the wire threaded beforehand through the wire passages and an initial hole formed in the workpiece for machining, to thereby effect electric discharge machining. They are further arranged to inject machining fluid from upper and lower nozzles, respectively provided at the upper and lower wire guides, toward an electric discharge machining region.
In electric discharge machines of this type, disconnection of the wire may occur during electric discharge machining. In the case of manufacturing plural products from one workpiece, moreover, the wire is intentionally cut every time the machining operation for one product is completed. In this respect, to extend the wire that has been disconnected by accident or intentionally between the two wire guides, the electric discharge machine is provided with an automatic wire extension apparatus for threading the wire fed through the upper wire guide while the wire is restricted by the machining fluid jetted from a wire extension nozzle attached to the upper nozzle into the initial hole of the workpiece and then into the wire passage of the lower wire guide.
In such wire extension apparatuses, generally, wire extension is more likely to be successfully carried out as the diameter of the initial hole is increased. In the case of manufacturing ordinary products by electric discharge machining, initial holes with a relatively large diameter may be formed, and therefore, wire extension can be achieved at a practically satisfactory rate. However, when manufacturing products requiring extremely precise machining, such as lead frames of integrated circuits, the diameter of the initial hole must be, in some cases, be smaller than the nozzle hole diameter of the wire extension nozzle. In such cases, the diameter of the nozzle hole (the diameter of the machining fluid jet) is larger than that of the initial hole, whereas the wire, when fed toward the initial hole, is displaceable in the diametrical direction of the jet despite the restriction by the jet. As a result, the wire is likely to be deviated from the initial hole in the diametrical direction of the jet (initial hole) when it reaches the upper surface of the workpiece, frequently causing the situation that the wire strikes against the upper surface of the workpiece and thus wire extension ends in failure.
In view of this, the diameter of the jet may be made smaller than that of the initial hole, by using a wire extension nozzle having a smaller nozzle hole diameter. If the diameter of the machining fluid jet (the diameter of the nozzle hole) is reduced, however, the area in the horizontal sectional area of the jet occupied by the wire increases. Therefore, the resistance which the machining fluid receives when passing through the initial hole is increased, resulting in an increase in the machining fluid pressure. For instance, when the wire diameter is 0.2 mm and the diameter of the nozzle hole is 0.3 mm, the machining fluid pressure is about 43 times as high as that when the diameter of the nozzle hole is 1.5 mm. If the machining fluid pressure is increased, the machining fluid leaks from between the upper nozzle and the wire extension nozzle. To prevent this, the fitting portions of these parts must be designed to be watertight and a high discharge pressure pump must be used for injecting the machining fluid, thus increasing the cost of the automatic wire extension apparatus. Further, wires having a diameter larger than the nozzle hole diameter are unusable, as they cannot be passed through the nozzle. If the diameter of the nozzle hole (the diameter of the jet) is small, the wire guiding function of the machining fluid jet is lowered, as mentioned above. Thus, by merely using a wire extension nozzle with a small nozzle hole to allow the wire to be inserted into an initial hole having a small diameter, disadvantages arise in that restriction is imposed on the diameters of wires usable for the manufacture of ordinary products, and that a required wire guiding function cannot be achieved by the machining fluid jet.